A compressed air processing system is known from German Patent No. DE 196 38 226 C1 corresponding to European Patent No. EP 0 831 383 B1. The housing of the known compressed air processing system includes an inlet connection to which a conduit coming from a processor is connected. A pressure control unit is arranged in the housing of the compressed air processing system. The pressure control unit includes a deaerating conduit leading to the atmosphere such that it switches between a load phase and an idle phase. In the load phase, compressed air flows into a central aerating system via an air dryer and a check valve. A central aerating system is to be understood as room or a space with which the respective conduits leading to the circuits of the system are connected. Each circuit includes an overflow valve being part of a multi-circuit protection valve. The overflow valve may also be generally designed as a pressure protection valve. The pressure protection valve may be designed as an electrically controllable solenoid valve. The system further includes a common electronic control unit serving to process signals and to control the pressure protection valves and additional valves such as valves being used during regeneration of the air dryer, for example. The known compressed air processing system also includes a parking brake connection, meaning a connection being connected to a conduit leading to a parking brake cylinder. This conduit branches off downstream of the pressure protection valve of a circuit, and it leads to the spring brake cylinders via a common hand brake valve. This conduit does not include a reservoir container since the amount of air required for release of the spring brake cylinder is comparatively small. The pressure control valve of this circuit from which the conduit leading to the parking brake connection branches off has a different design compared to the other pressure control valves. It includes a pneumatically controllable valve arrangement for aerating or locking the parking brake connection in a controlled way. The regeneration valve is also used to control the valve arrangement at the pressure control valve of this circuit. The valve arrangement may have two different positions, namely either the aerating position in which the conduit leading to the parking brake connection is aerated. The other position is the locking position in which this conduit is locked. The valve arrangement is controlled by the regeneration valve. The regeneration valve receives a control signal by the electronic control unit. Supply of the spring brake with compressed air is interrupted in the locking position such that it is possible to actuate the hand brake valve with its own deaerating system to actuate the parking brake. However, it is no longer possible to release the parking brake afterwards. This safety function may be of value in certain situations.
German Patent No. DE 196 38 226 C1 corresponding to European Patent No. EP 0 831 383 B1 discloses a second embodiment illustrated in FIG. 2 thereof. In this embodiment, the locking valve for the supply conduit leading to the compressed spring of the parking brake is not integrated in the compressed air processing system, but instead arranged outside of the system upstream of the hand brake valve. The electronic control unit of the known compressed air processing system includes an electric output connection transmitting a signal which may be transmitted to the electrically controllable locking valve. The locking valve then takes its locking position, while the passage position (or aerating position) is supplied by a mechanical spring. The known hand brake valve used in all modern brake systems which serves to aerate and deaerate the spring brake cylinder indirectly or directly requires a majority of conduits and conduit connections. Since the hand brake valve always has to be located in the driver's cabin of the motor vehicle, the respective conduits have to be installed in the driver's cabin which is comparatively complicated.
A compressed air processing system including an inlet connection being connected to a conduit coming from a compressor is known from German Patent Application No. DE 198 35 638 A1 corresponding to U.S. Pat. No. 6,540,308. The known compressed air processing system includes a pressure control unit, a multi-circuit protection valve and a plurality of outlet connections leading to the respective circuits. It also includes a parking brake connection connected to a conduit leading to a parking brake cylinder. The system also includes an electronic control unit. The compressed air processing system does not include a valve arrangement serving to aerate or lock the parking brake connection in a controlled way. A check valve may be integrated at the compressed air processing system to protect the parking brake system when a conduit in circuit III breaks. In the circuit III, meaning outside of the compressed air processing system, there may be a deaerating valve for the parking brake system.